Most of the heavy oil produced by steam injection techniques is obtained from wells operated in the cyclic, or "huff and puff" mode. Even those reservoirs under continuous injection, or steam flood, are usually started on production in the "huff and puff" mode, so as to develop first a hot zone of mobile oil in the reservoir in the immediate vicinity of the injection and production wells to be used later in the steam flood. This hot zone of mobile oil effectively increases the steam deliverability of the injection wells and the oil productivity of the production wells in the steam flood. This first step is particularly advantageous in the case of high-viscosity Heavy Oil reservoirs of relatively low permeability, where the injection of large volumes of steam would be precluded by the small effective radius of the wells. The creation of a hot mobile oil zone around the wells by the earlier "huff and puff" mode of operation gradually increases the effective radius of the wells. Another technique to address the same problem is to increase the surface of contact of the well with the reservoir, by using highly deviated or horizontal wells rather than vertical wells. These advantages are fully retained in the present process and several other advantages are added.
To reduce the heat loss from surface lines, these are insulated and mounted on supports above ground, whenever possible. This is, however, not possible in populated areas, where high pressure steam lines must be buried. Any thermal insulation in such buried lines must be protected from ground water and therefore requires two concentric pipes. This adds significantly to the cost of facilities when such surface lines are long, as in the case of conventional "huff and puff" operations in a field where wells are drilled on a relatively large spacing.
To reduce tubing heat losses, various thermal insulation techniques are also available, but their effectiveness is limited and their cost is high. This explains why steam injection techniques are presently limited to relatively shallow reservoirs. The present invention relaxes this depth limitation.
Large amounts of Heavy Oil have been discovered offshore under deep water and in the Arctic, under the Permafrost. In either case, heat losses through the well casing may be prohibitive, even when insulated tubings are used. This is why steam injection techniques have not been used in these cases, even when reservoir characteristics are favorable for the recovery of oil by application of such techniques. An embodiment of the present invention overcomes this problem.
The present invention takes advantage of the fact that, following a cycle of steam injection, the reservoir fluids produced in the "puff" part of the "huff and puff" mode of operation are very hot when they enter the bottom of the tubing, on their way to the surface. These produced fluids consist of hot oil, steam condensate and formation water, and gas associated with the oil. Their temperature is lower than that of the injected steam but it is still much higher than that of the rocks surrounding the well casing. The present invention provides the means of having, within the same casing, two tubings respectively carrying steam in downward flow and produced fluids in upward flow at the same time and exchanging heat between each other, as a way of offsetting the heat loss from the steam tubing to the surrounding casing and rocks.
In conventional "huff and puff" operations, the same well tubing, which may or may not be insulated, is used successively to transport steam in the first part of the cycle and produced fluids in the second part of the cycle, in discontinuous flow, first downward and later upward. Due to the discontinuous nature of the steam injection, the same boiler usually serves several injection wells, by means of a network of steam lines at the surface, which contribute to the degradation of steam quality.
On the contrary, in this novel process, two tubings, located within the same well casing, remain dedicated each to only one fluid, flowing continuously in a single direction. The steam tubing is always insulated and the temperature on the outer surface of its insulation is determined by that of the hot produced fluids flowing upwards rather than by that of the casing and surrounding rocks.
This reduces the temperature gradient across the insulation, so as to minimize the heat loss from the steam across the insulation layer. The unavoidable heat loss to the casing and to the cement and cold surrounding rocks is supplied mainly from the heat of the produced fluids stream. Consequently the steam quality of the injectant mixture arriving at the bottom of the steam tubing is much greater than in the conventional "huff and puff" process. This results in an increase of the oil/steam ratio, which is the essential economic parameter of any steam injection process.
This desirable result, which allows the continuation of oil production from wells otherwise uneconomic under the conventional mode of operation, also allows to produce heavy oil from deeper reservoirs for which conventional techniques would lead to excessive degradation of the steam quality and from offshore wells at great water depth, which are presently unexploited. It is achieved by using at least two horizontal drainholes, connected to the same vertical cased well and by using a novel Downhole Valve Section of the type described in the companion application Ser. No. 510,596, filed on Apr. 18, 1990. This Section consists of a set of tubular flow paths connected respectively to the drainholes at one end and to the tubular goods respectively carrying produced fluids and steam, at the other end of said Section. Novel retrievable multi-way downhole valves located within these flow paths and controlled from the surface allow to connect each of the drainholes to either the steam tubing or the production tubing and vice versa in successive cycles, so that each drainhole operates in the "huff and puff" mode while each tubing remains dedicated to the flow of the same type of fluids in a constant direction, on a continuous basis.
The same process and equipment, which includes boiler and production facilities preferably dedicated to a single vertical well and its connected drainholes, largely reduces the need for long steam lines and flow lines at the surface, thus further reducing heat losses and capital costs per barrel of oil produced.
Following a period of "huff and puff" operation, the drainholes within the field may be easily converted to the steam flood mode of operation, without any change in facilities.
Finally, an important advantage of the present process over the conventional "huff and puff" steam injection techniques is that the temperature of all the tubular goods in the vertical well remains essentially constant while the drainholes are in the "huff and puff" mode. This greatly reduces the maintenance cost of the well, because most of the expense in the conventional process is directly attributable to the periodic variations in thermal stresses and in thermal expansion of those tubular goods. Such variations are the source of leaks at the well head and in both the surface steam and production flow lines, requiring constant monitoring and frequent repairs.
Oil-soluble gases and/or foam additives may be used in conjunction with steam in the present process to provide additional oil recovery, according to known processes.